Too many UV mapping point
In my experience, most people are not going to UV map a mesh that dense.  That qualifies as high poly.
You have a few options to create a lower poly mesh that you can be more easily UV mapped.  The first and most important is manual retopology.  For a mesh like this, where we can expect little in the way of deformation, automatic remeshing tools can do the job in lieu of manual retopo.  Blender implements a few remeshers, in the way of the voxel and quad remesher tools, available in properties/object data/remesh, but they're not great.  Other alternatives include Instamesh and the paid addon Quad Remesher, which is the best in my opinion.
If you are certain that you do not want to retopo/remesh, there are some techniques that can simplify UV mapping of a dense mesh like this.  The first is select shortest path.  For me, that is select a vertex, ctrl select a distant vertex, and Blender will select all vertices along the shortest path between those two.  This can make seaming a breeze, after which, you just have to sit through however long the unwrap takes.
Of course, smart UV project unwraps will generally give you a usable UV map without any work, just a lot of CPU time.  Same with lightmap projects, although lightmap project can introduce really terrible UV distortion.
A final alternative is to make a quick, low poly cage mesh, UV map that, and then use a data transfer modifier to copy its UV map to your high poly.  This is almost always going to require some cleanup at seams.  You'll probably want to create a cage like this anyways, so that you can map your image to it and use it to do some selected-to-active texture baking (opposite of typical, baking low-to-high.)